Seaweed has been used as a fertiliser and soil conditioner for many years. Seaweed has been used in milled or finely divided form and in the form of a liquid extract. In general liquid extracts are preferred as they may be purchased as an aqueous concentrate, readily diluted to the required concentration and applied using agricultural spray equipment.
Seaweed extracts have been shown to increase crop yields, resistance of plants to frost, uptake of inorganic soil nutrients, resistance to environmental stress conditions, resistance to some pests such as red spider mite and aphids and to reduce fruit losses during storage. Seaweed extracts improve the root system of plants and increase their reproductive capacity, increasing the number of flowers per plant as well as the number of flowers per flowering season.
Seaweed species are grouped into red, green and brown seaweeds. The species used to produce extracts varies throughout the world. Most commercially available seaweed fertilisers, also called biostimulants, are made from brown seaweeds. Brown seaweeds are more prevalent in cold water regions of the world. In Australia, seaweed fertiliser products are mostly manufactured from alginate rich Bull Kelp or a blend with other seaweed.
Seaweed species vary in the amount of alginate present. The extraction process may also have a bearing on the amount and structure of alginate in the extract prepared from the seaweed. Alginates are important compounds in providing the soil conditioning properties of seaweed fertilisers and hence their presence in seaweed extracts is regarded as beneficial.
The presence of alginate has however placed severe limitations on the use of other plant nutrients with seaweed extracts and in particular the use of calcium salts with seaweed extract.
Calcium is widely recognized for its important function in plants to increase nutrient uptake, build strong cell walls for sturdier plants and increase vitality. Maintaining high levels of bio-available calcium in soil is critical in achieving adequate levels of calcium uptake. Calcium plays a very important role in plant growth and nutrition, as well as in cell wall deposition. The primary roles of calcium as a soil amendment are to help maintain chemical balance in the soil, reduce soil salinity, and improve water penetration. Calcium also plays a critical metabolic role in carbohydrate removal and neutralizing cell acids. It is important to note that even though many soils contain high levels of insoluble calcium such as calcium carbonate, crops grown in these soils will often show symptoms of a calcium deficiency. Calcium also has roles in mitigating plant stresses such as cold and heat.
Balancing the requirement of high biostimulant efficacy with high levels of calcium and alginate content in the seaweed extract is a problem. When seaweed extract containing alginate is combined with calcium salts an insoluble gel is formed which produces residues which may interrupt, block and interfere with agricultural equipment such as spraying and irrigation equipment. The presence of the gel residue also gives rise to poor product stability which is exacerbated in cold weather conditions. Importantly, the unwanted gel residue is not reversible in nature. Consumers also find the presence of gels unattractive and there is a significant risk that a non-homogeneous composition will result in under-dosing of the components or a loss of a portion of the composition through failure to adequately disperse solids prior to dosing.
Previous attempts to address this problem have lead to the use of separate seaweed and calcium fertiliser components which are combined in very dilute compositions prior to spraying. The potential problems are minimised by contact of calcium and alginate species only in very dilute compositions, such as when concentrates are diluted by, for example, dispersing one part by volume of concentrate in 200 parts by volume of water. This need to combine different materials adds to the cost of transport and handling of different compositions and also relies on appropriate dosing at the point of use. Others have obviated the problem by preparing solid particulate fertilisers which can be broadcast as solid fertilisers. This, however, loses the benefit of spray application which is a significant benefit of liquid seaweed extracts, furthermore when plants have a calcium deficiency, rapid uptake approaches such as foliar application mandate a liquid product. These types of deficiencies are inefficiently addressed via a slow release solid form product.
One example of a seaweed extract containing a calcium additive is disclosed in EP0884293 which teaches a combination of at least one natural or synthetic aminopurine derivative or an algal extract rich in such derivatives, and a source of calcium as an agent for stimulating the germination process and the development of the plant production structures. The sources of calcium may include limestone, calcium chloride and calcium salts of amino acids. This patent also teaches that aminopurine-rich extracts were obtained by extraction at pH 3 with sulphuric acid. The end-product (see Example 3 of EP0884293) is typically a solid.
A further example of a solid composition of a seaweed extract containing calcium is disclosed in EP 2268594. A water-soluble fertiliser suspension with calcium, soluble seaweed, and fulvic acid is processed into a solid powder form together with magnesium and phosphate in defined ranges.
The present inventors have examined the use of calcium chelation agents such as EDTA, fulvic acid, and amino acids in attempt to reduce the problem of gel formation on combining a liquid seaweed extract concentrate with a calcium source. Such strategies do not, however, lead to satisfactory liquid products when the liquid concentrate contains significant levels of alginate based on the weight of the concentrate composition.
It is desirable to have a one-pack liquid seaweed extract concentrate such as extract derived from one or more of Bull Kelp (Durvillae potatorum), Knotted Kelp (Ascophyllum nosodum) and Durvillae species D. antarctica that also contain calcium ions.